Conventionally, a thermal-type air flow measuring device for measuring an air flow rate by use of heat transfer from air is widely known. For example, the air flow measuring device is disposed in an intake passage of an internal combustion engine and used for measuring a flow rate of intake air suctioned into the engine (which can hereinafter be referred to as an intake air amount).
More specifically, this air flow measuring device takes in a part of an intake air mainstream flowing through the intake passage and generates an electrical signal in accordance with the intake air amount. The air flow measuring device includes a housing that defines a bypass flow passage through which the taken-in intake air flows, and a sensor that is accommodated in the bypass flow passage to produce the electrical signal as a result of heat transfer with the taken-in intake air. The air flow measuring device reduces the influence of turbulence of intake air mainstream in the intake passage by arranging the sensor in the bypass flow passage instead of disposing the sensor directly in the intake passage, through which the intake air mainstream passes, so as to output a measurement value with few variations.
In addition, pulsation is inevitably caused in the intake air mainstream in accordance with opening and closing of a valve of the engine. Accordingly, the intake air amount changes over time, fluctuating between a larger-side peak value of the pulsation and a smaller-side peak value of the pulsation. As a result, due to the thermal-type measuring method whereby the measured value is outputted using the heat transfer with air, the measured value is lower than a central value of the pulsation as a true value, and a negative-side error is thereby produced. Consequently, the air flow measuring device resolves the negative-side error of the measurement value by making longer a passage length of the bypass flow passage than a passage length when air flows straight through the intake passage without flowing through the bypass flow passage.
The pulsation of intake air tends to have a large amplitude along with, for example, the recent popularization of exhaust gas recirculation (EGR). When the amplitude of pulsation becomes large, a backward flow is periodically generated. When the intake air pulsation becomes great to such an extent that the backward flow is produced, if the sensor that cannot discern between the backward flow and forward flow is disposed, a detection is made on a positive-side for the backward flow as well. Thus, a positive-side error is caused. Even if the sensor that can distinguish between the backward flow and forward flow is disposed, the negative-side error becomes great. Therefore, the bypass flow passage having a longer passage length is required in order to eliminate the negative-side error.
Accordingly, an air flow measuring device, in which a structure for limiting a flow of the backward flow into the bypass flow passage is provided on a downstream side of an outlet of the bypass flow passage, is described in, for example, JP-A-H06-307906. However, in the air flow measuring device described in JP-A-H06-307906, the outlet of the bypass flow passage is parallel to the intake air mainstream, and a normal line of the outlet surface is perpendicular to the intake air mainstream. As a result, even though the intake air, which has passed through the bypass flow passage, flows out of the bypass flow passage through the outlet, it does not easily merge smoothly into the intake air mainstream. Furthermore, since the normal line of the outlet surface of the bypass flow passage is perpendicular to the intake air mainstream, in the vicinity of the outlet of the bypass flow passage, the flow of the intake air mainstream is locally bent by the flow out of the bypass flow passage, and a pressure drop of the intake air mainstream thereby becomes great.
Moreover, because the outlet of the bypass flow passage in JP-A-H06-307906 is parallel to the intake air mainstream, it primarily has a structure into which the backward flow does not easily enter, and entry of air due to the disturbance of the intake air mainstream is significant rather than the entry of the backward flow. For this reason, the air flow measuring device of JP-A-H06-307906 may have a large error because of the disturbance of the intake air mainstream. In addition, the air flow measuring device of JP-A-H06-307906 is a duct-integrated device, in which a pipe member that defines the intake passage, and a housing that defines the bypass flow passage are combined together. Accordingly, the device itself grows in size, and its dimensional tolerance increases, so that variation in performance is great among the products.